I've tried changing jacks to plastic ones with no success, I've also tried DC heaters and I get almost the same amount of hum (as I mentioned before hum is heard only in the distortion channel not in the clean channel), I've tried different amps as well and nothing. However after some tweaking I noticed that:

If I turn the guitar volume down the hum level remains constant, the weird thing is that hum is heard as it is (sounds like pure 60 Hz hum), it doesn't seem to be affected by the distortion effect.

It is not motorboating, it sounds like 60 cycle hum.

Another weird thing I noticed is that if I touch the cable that goes from anode to the decoupling cap (C25) the hum level remains constant, but if I touch the cable after the decoupling cap (C25) the hum level is increased, and there are some portions of the cable where the hum level remains constant and some really sensitive ones (near the decoupling cap).

I know that some hum is normal, but I've heard other tube based preamps with more than one gain stage that are really quiet, so I'm still not satisfied with mine. Any advice will be appreciated guys, thanks in advance.